The present invention relates to button-type electrochemical cells, and more particularly to button-type electrochemical cells free of added mercury.
Air electrochemical cells require a supply of oxygen to perform adequately and button air cells are used extensively in such devices as hearing aids. Typical commercial button-type alkaline electrochemical cells have a negative electrode (anode) with zinc or a zinc alloy as the active material and an alkaline electrolyte, such as potassium hydroxide. To reduce leakage, especially caused by gassing inside the cell during the electrochemical reaction, mercury has been added to the cell. Button cells on the market today employ a small amount of mercury. However, it has become apparent that mercury can be hazardous to the environment and to the health of humans and animals. Therefore, there has been an increased demand by the public and federal, state, and local governments to substantially decrease or eliminate mercury in all electrochemical cells, including button-type cells.
The elimination of mercury without making any other changes to the commercially-available button cell results in cell leakage caused by one or a combination of the following: (1) gassing, (2) capillary action of the sealing areas, (3) electrochemical creepage driven by the potential difference within the cell and (4) a cracked or delaminated hydrophobic (e.g., polytetrafluoroethylene) layer. Furthermore, pressure within button air cells can cause delamination of the hydrophobic layer from the air electrode, which creates void space, causing electrolyte to accumulate therein. This accumulation of electrolyte results in a barrier for air to reach the air electrode of the cell.
Approaches have been taken in an attempt to eliminate the leakage problem in the button-type cells, while substantially decreasing or eliminating the mercury content of the cell at the same time. Mansfield, Jr. et al. U.S. Pat. Nos. 5,306,580 and 5,279,905 disclose an air cell with an indium electroplated inner surface of a refold-type anode cup. However, it was found that although the indium reduces gassing, the indium on the inner surface of the anode cup results in an unacceptable level of salting for commercially available button air cells that employ the indium electroplating. This is a result of increased capillary action, due to the greater roughness of the indium compared to copper, even though gassing is lower on an indium vs. copper surface.
Japanese laid-open patent application 10-162869 to Toshiba Battery Company, Ltd. discloses an unamalgamated air battery having a refold type anode cup made of a triclad (nickel, stainless steel, and copper) layered composition. The anode uses a zinc powder with a specific bulk density range and electrolytic solution ratio to attempt to increase the discharge utilization rate and increase leakage resistance. The teachings of this reference alone have not been successfully applied to button cells without producing unacceptably high levels of salting.
In U.S. Pat. No. 5,168,018, Yoshizawa et al. disclose the addition of an inorganic inhibitor, such as In(OH)3, and an organic surfactant, such as perfluoroalkyl polyethylene oxide, to a corrosion-resistant zinc alloy-containing anode of an alkaline cell with no added mercury to reduce hydrogen gas generation. They infer that part of the In(OH)3 is electrodeposited onto the surface of the zinc alloy, raising the hydrogen overvoltage on the surface of the zinc alloy, while the remaining In(OH)3 remains in the alkaline electrolyte until it is electrodeposited onto the surface of the zinc alloy that is freshly exposed during cell discharge. The surfactant forms a hydrophobic monomolecular layer on the surface of the zinc alloy to make the zinc alloy even more corrosion resistant. The teachings of Yoshizawa et al. alone have not been successfully applied to button cells without producing unacceptably high levels of leakage.
There is a need for a zero mercury, button-type electrochemical cell that is free of added mercury, highly resistant to leakage and salting, yet cost-effective to manufacture and commercialize, and provides satisfactory electrical performance.
The present invention is a button-type electrochemical cell, preferably a button air cell, comprising an anode cup having a sealing surface for contact with a gasket or seal, where the sealing surface of the anode cup has thereon no non-in situ-deposited indium, and preferably no non-in situ-deposited metal with a hydrogen overvoltage higher than that of copper. The electrochemical cell of the present invention is free of added mercury and has an anode with an active material that preferably comprises low gassing zinc. Preferably indium hydroxide is added to the anode mixture. If indium or another metal with a higher hydrogen overvoltage is put onto the interior surface of anode cups used to make cells according to the present invention, the indium or other metal is restricted to portions of the inner surface of the anode cup that are not in the seal area.
As used herein, the seal area of the anode cup is that portion of the surfaces of the anode cup that is in contact with the gasket or seal to form a seal between the anode cup and the gasket.
As used herein, in situ-deposited metal is metal that is deposited on the surface of the anode cup, as a result of contact with the anode mixture and/or electrolyte, after combining the anode cup with the anode mixture and/or electrolyte. Non-in situ-deposited metal is metal that is on the surface of the anode cup, or the material from which the anode cup is made, before the anode cup is combined with the anode mixture and/or electrolyte (e.g., as a result of cladding, coating, electrolytic plating, etc).
As used herein, an anode cup xe2x80x9chaving no non-in situ-deposited indium (or metal with a hydrogen overvoltage than that of copper) thereonxe2x80x9d is an anode cup having no indium (or metal with a hydrogen overvoltage higher than that of copper) on its surface that was present on that surface before the anode cup was combined with the anode mixture and/or electrolyte. The surface may, however, be a metal which includes indium (or a metal with a hydrogen overvoltage greater than that of copper) as an impurity or as a minor ingredient (i.e., less than about 100 parts per million (ppm) by weight) in an alloy.
Another aspect of the present invention is the method of forming an electrochemical cell comprising the steps of: (a) forming an anode with an active material comprising zinc; (b) providing an anode cup with an inner surface, comprising an area, free of indium or any other metal with a hydrogen overvoltage higher than that of copper, for sealing against a gasket; (c) cleaning the inner surface of the anode cup; (d) providing an electrolyte, an air electrode, and a cathode can; and (e) combining the anode, air electrode, electrolyte, cleaned anode cup, and the cathode can to form a button-type air electrochemical cell. The cell that is formed is free of added mercury.
These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims, and appended drawings.